#进阶1:基础查询 /* 语法: SELECT 查询列表 FROM 表名; 特点: 1、查询列表可以是:表中字段、常量值、表达式、函数 2、查询的结果是一个虚拟的表格 */ USE myemployees; #1、查询单个字段 SELECT last_name FROM employees; #2、查询多个字段 SELECT last_name, salary, email FROM employees; #3、查询所有字段 SELECT * # 顺序和原表一致 FROM employees; #4、查询常量值 SELECT 100; SELECT 'wangchaojun'; #5、查询表达式 SELECT 100%98; #6、查询函数 SELECT VERSION(); #7、起别名 #方式一: SELECT 100%98 AS res; SELECT last_name AS 姓, first_name AS 名 FROM employees; #方式2 SELECT last_name 姓 FROM employees; SELECT salary AS "out put" #如果占了关键字,要加引号 FROM employees; #8、去重 #案例:查询员工表中涉及到的所有部门编号 SELECT DISTINCT department_id FROM employees; #9、+号的作用(仅能做数值型的运算符) #案例:查询员工名和姓连接成一个字段 SELECT Concat(last_name, ' ', first_name) AS 姓名 FROM employees; SELECT IFNULL(commission_pct, 0) AS 奖金率, commission_pct FROM employees; SELECT Concat(last_name, ' ', first_name, ',', IFNULL(commission_pct, 0)) AS OUT_PUT FROM employees; #显示表结构 DESC departments;